1. Field of the Invention
This invention relates to an improved roof anchor which is partially retained in a rock formation by resin bonding material which has an axially adjustable resin retaining washer that is positionable along the shaft of the roof anchor to retain the resin at the end of the roof anchor and exert a compressive force on the resin before it sets.
This invention also relates to a method and apparatus for increasing the horizontal force produced by a mechanical expansion assembly attached to a mine roof bolt. The invention also reduces the likelihood that the mechanical expansion shell will fail to grip the walls of the bore hole and includes lubricating the surfaces of the leaves of the mechanical expansion shell or the inner plug which expands the shell or both.
2. Description of the Prior Art
It is well known in the art of mine roof support to tension roof anchors in bore holes drilled in the mine roof to reinforce the unsupported rock formation above the roof. Conventionally, a hole is drilled through the roof into the rock formation. The end of the roof anchor is anchored in the rock using either a mechanical expansion shell to grip the walls of the bore hole or by adhesively bonding the anchor with a resin bonding material to the rock formation surrounding the bore hole or by using a combination of a mechanical expansion shell and resin bonding material. When resin bonding material is used, it penetrates the surrounding rock formation to adhesively unite the rock strata and to firmly hold the roof anchor in position within the bore hole. The resin mixture fills the annular area between the bore hole wall and the shaft of the roof anchor.
When a mechanical expansion shell is used in a wet bore hole or together with resin, a problem often develops with "spinners". "Spinners" are mechanical expansion shells which do not expand sufficiently to grip the walls of the bore hole and anchor the bolt assembly within the bore hole.
U.S. Pat. No. 4,419,805 and U.S. Pat. No. 4,413,930 are examples of mine roof anchors utilizing combination of an expansion shell and a resin bonding material to retain the roof anchor within the rock strata. These patents disclose rigid resin retaining washers which are axially fixed to the shaft of the roof anchor and which may not be adjusted after the roof anchor leaves the manufacturing site and is delivered to the mine site.
They also disclose the use of resin to prevent the slippage of the mechanical expansion shell in rock formations such as shale, sandstone, and mudstone after deterioration of the rock formation surrounding the expanding shell has reduced the contact area between the shell and the rock formation.
U.S. Pat. No. 4,162,133 also discloses a roof anchor which is retained within a rock strata by both a mechanical expansion anchor and resin bonding material. This patent discloses a rigid resin retaining washer that is supported on the shaft of the roof anchor by ears that are pinched into the shaft of the roof anchor in a fixed position. The rigid resin supporting washer of this patent is not axially adjustable since the position of the ears pinched into the shaft of the roof anchor will determine the position that the rigid washer assumes when the resin comes into contact with it.
A publication by Bethlehem Steel Company entitled "Bethlehem Roof and Rock Bolts" discloses a resin-anchored roof bolt that has a retaining washer mounted thereon. The bolt is threaded at both ends and the steel washer retains the mixed resin in the bolt hole and also compacts the mixed resin.
U.S. Pat. No. 4,746,248 discloses an anchor bolt anchored by means of resin bonding. Hot-melt adhesive compositions are used as lubricants between the washer and the bolt head and are activated by heat as the bolt is rotated and tensioned in the bore hole. This patent does not disclose the use of mechanical expansion shells.
U.S. Pat. No. 4,619,559 discloses a friction reducing washer between a nut and the roof plate. The nut is threadedly secured to the end of a bolt extending from the bore hole and the friction reducing washer is utilized to increase the tension on the bolt.
U.S. Pat. No. 4,305,687 discloses the use of a roof anchor system consisting of a roof bolt with one or more wedge portions formed on its upper end. The anchor is secured in place by resin bonding material which is forced into contact with the walls of the hole by the wedged-shaped portion of the anchor. It is stated in this patent that compression is best achieved if the wedged-shaped portions of the bolt do not bond well to the grout. Therefore, coatings such as foil coatings, organic coatings or bonded lubricants such as teflon or molybdenum disulfide are placed on the wedge to allow it to move within the resin without being bound to the resin.
We have found that by providing an axially adjustable rigid resin retaining washer we can accurately coordinate the annular area available for the resin to occupy with the amount of resin that is utilized in the roof anchor system so that when the roof anchor is installed, the upward thrust of the roof anchor will exert a hydraulic force on the resin bonding material to confine it within a restricted annular area at the end of the roof anchor and cause the resin bonding material to be forcefully driven into the cracks and crevices on the inside wall of the bore hole and into the surrounding rock formation to more solidly lock the roof anchor within the rock formation.
We have also found that a lubricated expansion shell assembly increases substantially the horizontal force transmitted to the leaves of the mechanical expansion shell as they grip the walls of the bore hole.